AP Physics 2 : Circuit Power

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #262 : Electricity And Magnetism

A single battery is in series with a  resistor. How would the output of the circuit change if a second  was added in series?

Possible Answers:

Doubled

Tripled

Quadrupled

Quartered

Halved

Correct answer:

Halved

Explanation:

The power dissipated by the resistor is

Using Ohm's Law

And changing the equation to be exclusively in terms of voltage and resistance

From this, it can be seen that doubling the resistance will halve the power dissipated if the voltage is kept constant.

Example Question #21 : Circuit Power

Vt physics 11 22 15 circuit 1

Vt physics 11 22 15 circuit 2

Vt physics 11 22 15 circuit 3

Consider the three circuits shown. In each circuit, the voltage source is the same, and all resistors have the same value. If each resistor represents a light bulb, which of the three circuits will produce the brightest light?

Possible Answers:

Circuits A and B will produce the same (highest) brightness, with circuit C producing the lowest brightness

Circuit A

Circuit B

Circuit C

Circuits A, B, and C will all produce brightness of the same intensity

Correct answer:

Circuit A

Explanation:

To answer this question, we first need to determine what we're looking for that will allow any given light bulb to produce bright light. The answer is power. The more energy that is delivered to the light bulb within a given amount of time, the brighter the light will be. Let's go ahead and look at the equation for power in a circuit.

And since we're told in the question stem that the voltage source is identical in each circuit, we're looking for the circuit that has the largest current.

In order to find which circuit has the largest current, we'll need to invoke Ohm's law.

What this shows is that a higher current will occur in the circuit that has the lowest total resistance. Thus, we'll need to determine what the total resistance is in each of the three circuits.

In circuit A, we can see that there is only a single resistor. Thus, we can give this circuit's total resistance a value of .

In circuit B, we have two resistors that are connected in series. Remember that when resistors are connected in this way, the overall resistance of the circuit increases. We find the total resistance by summing all the resistors connected in series.

Therefore, we can give circuit B a total resistance of .

Now, let's look at circuit C. We can see that there are two resistors connected in parallel, and each of these are connected in series with a third resistor. To solve for the total resistance of this circuit, we first need to determine the equivalent resistance of the two resistors connected in parallel. Once we find that value, then we can take into account the third resistor connected in series.

To solve for the resistance of the two resistors connected in parallel, we have to remember that they add inversely.

Now that we've found the equivalent resistance for the two resistors connected in parallel, we can consider the third resistor connected in series.

Thus, we can give circuit C a value of .

Now that we've found the total resistance for each circuit, we can obtain our answer. Since circuit A has the lowest total resistance, it will also have the greatest current. Consequently, it will also have the greatest power delivered to its resistor (the light bulb), thus causing the light coming from that bulb to be the brightest.

Example Question #23 : Circuit Power

A  battery is placed in series with five  resistors. Find the total power dissipated by the circuit.

Possible Answers:

Correct answer:

Explanation:

In series, resistance adds conventionally.

Using Ohm's law:

Using definition of electric power:

Plugging in values:

Example Question #23 : Circuit Power

A single  resistor is added in series to a circuit with a  battery. Determine the power dissipated by the resistor.

Possible Answers:

Correct answer:

Explanation:

Use Ohm's law: 

Converting  to 

Using definition of electric power:

Plugging in values:

Example Question #25 : Circuit Power

A lone resistor is placed in series with a battery. How will adding a second resistor in series affect the power output of the circuit?

Possible Answers:

Cut it in half

Double it

None of these

It will be unaffected

Quadruple it

Correct answer:

Cut it in half

Explanation:

Definition of electrical power:

Ohm's law:

Combining equations:

When adding resistors in series, they add directly

Thus, in this case, resistance would increase, and if the voltage is kept constant, the power dissipated by the circuit would decrease. Doubling the resistance would cut the power output in half.

Example Question #24 : Circuit Power

A lone resistor is placed in series with a battery. How will adding a second resistor in parallel affect the power output of the circuit?

Possible Answers:

Quadruple it

Double it

Cut it in half

None of these

It would stay the same

Correct answer:

Double it

Explanation:

Definition of electrical power:

Ohm's law:

Combining equations:

When adding resistors in parallel, they add inversely

Thus, in this case, resistance would decrease, and if the voltage is kept constant, the power dissipated by the circuit would increase. Adding identical resistors in parallel would halve the resistance, which would double the power dissipated.

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